1. Field of the Invention
The invention relates to a voltage mode boost converter and, more particularly, to a voltage mode boost converter using a period-fixed amplitude-modulated pulse control signal.
2. Description of the Related Art
The voltage required by common semiconductor components or microelectronic devices is mostly between 3.0V and 5.5V, while the voltage source required by some devices may be larger; for example, the voltage for driving an LCD driver, or the voltage for a flash memory, which is mostly between 6V and 7V. Therefore, most industrial manufacturers provide a voltage mode boost converter to convert circuit voltage so that a lower voltage (3.0V-5.5V) can be stepped up (boosted) to a higher voltage (6V-7V) for use.
As shown in FIG. 1A, a conventional circuit configuration of a voltage mode boost converter 1 comprises a pulse-width modulation control circuit 11, a step-up (boost) circuit 12 and a feedback circuit 13.
As for the voltage mode boost converter 1, when it converts voltage, it must coordinate with a triangle wave generator (not shown), which is used to generate a triangle wave signal, and the triangle wave signal is input to the pulse-width modulation control circuit 11 to proceed with the pulse-width modulation control. As well known in the art, the pulse-width modulation control circuit 11 is operated in accordance with a clock.
The principle of the pulse-width modulation control is to use the feedback voltage signal VFB, which is generated by the feedback circuit 13, and to use the triangle wave signal to adjust the waveform duty time. FIG. 1B is a diagram illustrating how to use the feedback voltage signal VFB to adjust the waveform duty time, wherein Vrp and Vrn each represents a peak value of the amplitude of the triangle wave signal, and VFB1 and VFB2 each represents the feedback voltage signal at different times. As shown in FIG. 1B, when the feedback voltage signal VFB is at VFB1, its corresponding duty time is T1, and when the feedback voltage signal VFB is at VFB2, its corresponding duty time is T2. In brief, the above-mentioned pulse-width modulation control circuit 11, in accordance with the feedback voltage signal VFB and the triangle wave signal, can generate a pulse signal with an adjustable duty time, and the pulse signal is a signal that appears at the point O shown in FIG. 1A, while the waveform of the signal that appears at the point O is shown in FIG. 1B.
In addition, as shown in FIG. 1A, the step-up circuit 12 of the voltage mode boost converter 1 comprises a MOS device as a switch, an inductor device L for providing an electric charge, a diode device D for rectifying, and a capacitor device C for storing an electric charge. Among these devices, the MOS device, the diode device, and the capacitor device are connected to one another in series, and the inductor device is connected between the MOS device and the diode device in parallel. The signal (i.e., the signal that appears at the point O) output by the pulse-width modulation control circuit 11 is for controlling the gate of the MOS device so that the inductor device L can charge to the capacitor device C.
Also, the feedback circuit 13 comprises a resistor R1, a resistor R2 and a feedback terminal 131. The resistor R1 is connected to the resistor R2 in series, and one end of the feedback terminal 131 is electrically connected between the resistor R1 and the resistor R2, while the other end of the feedback terminal 131 is electrically connected to the pulse-width modulation control circuit 11. The feedback circuit 13 is used to generate a feedback voltage signal VFB to the pulse-width modulation control circuit 11, and accordingly controls the output of the pulse-width modulation control circuit 11.
However, as for the voltage mode boost converter 1, it has a shortcoming that when stepping up (boosting) the voltage, it must rely on a triangle waveform generator to generate a triangle waveform signal; otherwise, the voltage mode boost converter 1 cannot operate smoothly, which means that the triangle waveform generator must keep on operating all the time. In other words, the voltage mode boost converter 1 cannot go into a standby mode.
As shown in FIG. 2A, another conventional voltage mode boost converter 2 comprises a pulse-frequency modulation control circuit 21, a step-up circuit 22 and a feedback circuit 23. As for the voltage mode boost converter 2, the circuit configuration and performance of its step-up circuit 22 and feedback circuit 23 are the same as those of the above-mentioned conventional voltage mode boost converter 1. The difference between the voltage mode boost converters 1 and 2 is that the latter uses a pulse-frequency modulation control circuit 21 to generate control signals. The pulse-frequency modulation control circuit 21 mainly utilizes the amplitude of the feedback voltage signal VFB, which is output by the feedback circuit 23, to adjust the period of the control signal that appears at the point O. As well known in the art, the pulse-frequency modulation control circuit 21 is operated in accordance with a clock. As shown in FIG. 2B, the larger the feedback voltage signal VFB output by the feedback circuit 23 is, the larger the period of the control signal that appears at the point O will be.
However, as for the voltage mode boost converter 2, it has a shortcoming that the voltage output by the voltage mode boost converter 2 is affected by the change of amplitude of VCC voltage, thereby causing poor quality output voltage.
In view of the shortcomings of the conventional voltage converters, to provide a voltage mode boost converter with a standby mode and good quality output voltage becomes an important task.
The object of the invention is to provide a voltage mode boost converter, which has a standby mode and the feature of low standby current, and has good quality output voltage.
The feature of the invention is to provide a control signal generating circuit, which uses a feedback voltage signal to generate a step-up control pulse signal that is period-fixed and amplitude-modulated, so that the voltage mode boost converter of the invention can have a standby mode and a low standby current, and also ensure good quality output voltage.
Thus, to achieve the above-mentioned objective, a voltage mode boost converter in accordance with the invention comprises a control signal generating circuit, which is used to generate a step-up control pulse signal that is period-fixed and amplitude-modulated, a step-up circuit, which adjusts the output voltage signal of the step-up circuit according to the step-up control pulse signal generated by the control signal generating circuit, and a feedback circuit, which generates a feedback voltage signal according to the output voltage signal of the step-up circuit. The feedback voltage signal is input into the control signal generating circuit, and the control signal generating circuit generates the step-up control pulse signal according to the feedback voltage signal.